RFP_Finanzen/src/lib/recurring-expenses.ts

export type RecurringOccurrence = {
  id: string;
  label: string;
  dueAt: string;
  amount: number;
};

const monthLabelFormatter = new Intl.DateTimeFormat("de-DE", {
  month: "long",
  year: "numeric"
});

function toDate(value: string | Date) {
  return value instanceof Date ? value : new Date(value);
}

function startOfUtcDay(date: Date) {
  return new Date(Date.UTC(date.getUTCFullYear(), date.getUTCMonth(), date.getUTCDate(), 0, 0, 0, 0));
}

function endOfUtcDay(date: Date) {
  return new Date(Date.UTC(date.getUTCFullYear(), date.getUTCMonth(), date.getUTCDate(), 23, 59, 59, 999));
}

function startOfUtcMonth(date: Date) {
  return new Date(Date.UTC(date.getUTCFullYear(), date.getUTCMonth(), 1, 0, 0, 0, 0));
}

function addUtcMonths(date: Date, months: number) {
  return new Date(Date.UTC(date.getUTCFullYear(), date.getUTCMonth() + months, 1, 0, 0, 0, 0));
}

function daysInUtcMonth(year: number, monthIndex: number) {
  return new Date(Date.UTC(year, monthIndex + 1, 0)).getUTCDate();
}

function buildDueDate(monthCursor: Date, recurrenceStartAt: Date) {
  const year = monthCursor.getUTCFullYear();
  const monthIndex = monthCursor.getUTCMonth();
  const day = Math.min(recurrenceStartAt.getUTCDate(), daysInUtcMonth(year, monthIndex));

  return new Date(Date.UTC(year, monthIndex, day, 12, 0, 0, 0));
}

export function buildRecurringOccurrences({
  expenseId,
  amount,
  recurrenceStartAt,
  periodStartsAt,
  periodEndsAt
}: {
  expenseId: string;
  amount: number;
  recurrenceStartAt: string | Date;
  periodStartsAt: string | Date;
  periodEndsAt: string | Date;
}) {
  const seriesStart = startOfUtcDay(toDate(recurrenceStartAt));
  const periodStart = startOfUtcDay(toDate(periodStartsAt));
  const periodEnd = endOfUtcDay(toDate(periodEndsAt));

  if (seriesStart > periodEnd) {
    return [] as RecurringOccurrence[];
  }

  const occurrences: RecurringOccurrence[] = [];
  let cursor = startOfUtcMonth(seriesStart > periodStart ? seriesStart : periodStart);
  const lastMonth = startOfUtcMonth(periodEnd);

  while (cursor <= lastMonth) {
    const dueAt = buildDueDate(cursor, seriesStart);

    if (dueAt >= seriesStart && dueAt >= periodStart && dueAt <= periodEnd) {
      occurrences.push({
        id: `${expenseId}-${cursor.getUTCFullYear()}-${String(cursor.getUTCMonth() + 1).padStart(2, "0")}`,
        label: monthLabelFormatter.format(dueAt),
        dueAt: dueAt.toISOString(),
        amount
      });
    }

    cursor = addUtcMonths(cursor, 1);
  }

  return occurrences;
}

export function getExpensePeriodAmount(amount: number, recurrence: "NONE" | "MONTHLY", occurrenceCount: number) {
  return recurrence === "MONTHLY" ? amount * occurrenceCount : amount;
}